drm/amd/display: Eliminate i2c hw function pointers

[Why]
The function pointers of the dce_i2c_hw struct were never
accessed from outside dce_i2c_hw.c and had only one version.
As function pointers take up space and make debugging difficult,
and they are not needed in this case, they should be removed.

[How]
Remove the dce_i2c_hw_funcs struct and make static all
functions that were previously a part of it.  Reorder
the functions in dce_i2c_hw.c.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>